1. Field of the Invention
The invention concerns two-part polyurethane adhesives that provide final strength at relatively low post bake temperature conditions. The adhesive comprises a prepolymer having a reactive non-ionic surfactant and an allophanate modified isocyanate and a curative comprising less than 5 equivalent percent total amino and hydroxyl groups having an equivalent weight greater than about 200. The invention further concerns processes for adhering substrates with the adhesive composition.
2. The Related Art
Adhesives having initial solidification to adhere substrates to allow movement of the bonded substrates to a secondary post bake operation facilitate the manufacture of assembled parts and materials. Adhesives that accelerate the initial solidification will allow for more efficient production of parts.
For example, within the transportation market it is common practice to accelerate the initial solidification of an adhesive bond line by applying direct heat in a bond fixture, typically from about 82° C. to about 315° C., until a minimum strength is achieved to move the part, often referred to as green strength. Final adhesive strength is then achieved following a secondary post bake operation, generally at elevated temperatures, like those provided by a paint or electro-coat primer oven of about 149° C. to about 204° C.
Two challenges are present within the transportation industry, as well as other industries, involving the manufacture of materials having bonded parts, that may affect the performance of the adhesive used in bonding operations. First is the desire to minimize capital investment necessary to fabricate a bond fixture by the elimination of heat, and the second is the drive to reduce fixed cost by lowering the temperature of post bake ovens. With conventional adhesive technology, following room temperature cure with a reduced temperature post bake, such as in a temperature range of about 82° C. to about 104° C., the adhesive will solidify and provide bond strength and passing fiber tear failure pattern at room temperature, but the bond may fail in the undesirable adhesive failure mode during elevated temperature strength testing or in service use. Such bond failure can be eliminated or curtailed by sanding and/or priming the substrate, both requiring capital. Further, primerless, that is no surface preparation, two-part polyurethane adhesives have been mentioned in the art, however, the combination of materials making these primerless adhesives involve polymerizations requiring both high temperature fixture cure followed by a high temperature post bake to provide the requisite adhesion.
All parts and percentages set forth herein are on a weight-by-weight basis unless specified otherwise.